A study released on Sunday tallies the chemical weapons attacks over the course of the Syrian civil war, which has left hundreds of thousands dead. At least 336 have occurred, according to authors Tobias Schneider and Theresa Lütkefend of the Berlin-based Global Public Policy Institute.

That’s a number higher than previously known — and “we suspect that the real number may still be significantly higher,” they write.

The researchers pored over reports of chemical attacks going back to 2012, reviewing the available information on each to verify the details. They collected evidence from “Syrian and international non-governmental organizations, monitoring groups, private firms, local administrative bodies, relevant international bodies, local and international media, and the open source.”

The report’s authors attributed 2 percent of the chemical weapons attacks in Syria to the Islamic State.

Syrian President Bashar al-Assad’s regime carried out 98 percent of them, according to the authors, dropping chlorine gas, sarin and sulfur mustard gas on Syrian civilians.